How yeast sense direction in shallow pheromone gradients

这个项目将有助于我们对梯度感应的理解，即细胞感知其表面化学物质浓度的微小差异的能力，从而定位刺激的来源。这种现象对所有生物的发育和健康都是必不可少的。首席研究员使用酵母细胞作为模型来研究梯度感应的分子机制，这被认为广泛适用于更复杂生物体的细胞。在这个项目期间，首席研究员和他的博士后科学家将指导附近的马尔科姆·艾克斯学院（MXC），芝加哥城市学院之一的生物学学生，努力提高他们从四年制大学毕业并获得STEM领域学士学位的机会。拟议的本科研究和指导计划旨在激励、指导、建议和支持可能对STEM职业感兴趣的代表性不足的学生。该调查还将通过与不同领域的专家合作，为学生和博士后科学家提供跨学科的培训。由于该项目是数学和生物学的结合，每个团队的研究人员将更好地了解其他学科的研究人员使用的方法。最著名的梯度刺激细胞输出，趋化性（定向细胞运动）和趋化性（定向细胞生长），是广泛的生物过程所必需的。尽管趋化细胞和趋化细胞最终表现出截然不同的行为，但它们面临着相似的挑战：响应细胞必须感知其表面化学浓度的微小差异，确定梯度源的方向，并将其细胞骨架向其极化。出芽酵母的交配反应是趋化的：交配细胞解释复杂的信息素梯度，并向最接近的伴侣方向极化生长。像许多化学吸收细胞一样，酵母使用G蛋白偶联受体来检测潜在伴侣分泌的交配信息素，从而引导它们向最近的信息素来源生长。这个项目的目标是了解在细胞骨架极化之前，趋化生长位点是如何建立的，以及细胞如何对梯度方向的变化做出反应。已经提出了各种模型来解释酵母如何在体内解释浅层信息素梯度，但没有一个令人满意地回答了一个基本和长期存在的问题：细胞如何从用于细胞分裂的默认极性位点切换到建立趋化位点，尽管信噪比接近于零？根据之前项目的发现，首席研究员提出了一个模型来回答这个问题。工作假设是，交配酵母最初忽略了信息素梯度，因为它们首先在默认位置共定位并集中信号和运输蛋白质，建立了一个“梯度跟踪机器”（GTM）。一旦组装，GTM沿着质膜移动到信息素浓度最大的点，在那里它标志着趋化位点。这项研究的重点是了解GTM是如何组装的，它是如何移动到趋化位点的，以及它是如何根据梯度方向的变化来引导趋化生长的。这些问题将使用成像、遗传学、光遗传学、生化和计算方法来回答，从而更深入地了解化学梯度传感。由于我们对其他真核生物在趋化反应过程中如何建立梯度排列的细胞极性知之甚少，因此可能会出现一些普遍的原理，这些原理将对趋化现象的研究产生广泛的影响。此外，本研究中提出的许多问题与其他跨膜信号系统有关，研究结果有望揭示普遍相关的机制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

This project will contribute to our understanding of gradient sensing, the ability of cells to sense small differences in chemical concentration across their surfaces, and thereby locate the source of the stimulus. This phenomenon is essential for the development and health of all organisms. The principal investigator uses yeast cells as a model to study the molecular mechanisms underlying gradient sensing, which are thought to be broadly applicable to cells in more complex organisms as well. During this project period, the principal investigator and his postdoctoral scientist will mentor select biology students from nearby Malcolm X College (MXC), one of the City Colleges of Chicago, in an effort to enhance their chances of graduating from a four-year institution with a BS in a STEM field. The proposed undergraduate research and mentoring program is designed to inspire, instruct, advise, and support underrepresented students who may be interested in a STEM career. The investigation will also provide students and the postdoctoral scientist with interdisciplinary training through interactions with collaborators who are experts in diverse areas. Because the project is at the interface of math and biology, researchers on each team will gain a better understanding of the methods used by those in the other discipline.The best-known gradient-stimulated cellular outputs, chemotaxis (directed cell movement), and chemotropism (directed cell growth), are required for a wide range of biologic processes. Although they ultimately exhibit quite different behavior, chemotactic and chemotropic cells face similar challenges: the responding cell must sense small differences in chemical concentration across its surface, determine the direction of the gradient source, and polarize its cytoskeleton toward it. The mating response of the budding yeast S. cerevisiae is chemotropic: mating cells interpret complex pheromone gradients and polarize their growth in the direction of the closest partner. Like many chemotaxing cells, yeast use G protein-coupled receptors to detect mating pheromone secreted by potential partners and thereby direct their growth toward the nearest pheromone source. The goal of this project is to understand how the chemotropic growth site is established before polarization of the cytoskeleton, and how the cell responds to changes in gradient direction. Various models have been proposed to explain how yeast interpret shallow pheromone gradients in vivo, but none satisfactorily answers the fundamental and long-standing question: how do the cells switch from the default polarity site they use for cell division to establish a chemotropic site, despite a near zero signal-to-noise ratio? Based on discoveries made during a previous project, the principal investigator proposed a model that answers this question. The working hypothesis is that mating yeast initially ignore the pheromone gradient, as they first colocalize and concentrate signaling and trafficking proteins at the default site, building a "gradient tracking machine" (GTM). Once assembled, the GTM moves along the plasma membrane to the point of maximal pheromone concentration, where it marks the chemotropic site. The priorities of this investigation are to learn how the GTM is assembled, how it moves to the chemotropic site, and how it steers chemotropic growth in response to changes in gradient direction. These questions will be answered using imaging, genetic, optogenetic, biochemical, and computational approaches, leading to a deeper understanding of chemical gradient sensing. Because little is known about how gradient-aligned cell polarity is established during the chemotropic responses of other eukaryotes, general principles are likely to emerge that will broadly influence the study of chemotropic phenomena. Moreover, many of the questions posed in this investigation are pertinent to other transmembrane signaling systems, and the findings are expected to reveal generally relevant mechanisms.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.